Various laser procedures or operations require that the laser beam be properly focused to a specific focal point. For example, in ophthalmic laser surgery wherein eye tissue is to be photodisrupted or ablated in or on the tissue that is to be affected, the correct positioning of a focusing assembly used to focus a laser beam is very critical. Such ophthalmic surgical procedures include those in cornea, sclera, iris, the crystalline lens and related structures, vitreous, and retina, and for treatment of glaucoma. Focal depth precision is also required in many non-ophthalmic laser surgical procedures, such as applications in dermatology and even xe2x80x9csurgeryxe2x80x9d in DNA to excise portions of chromosomes. Also, non-biologic applications, such as photolithography and micromachining require focal depth precision.
Even with calibration of a focusing element for a laser, which is made to vary according to the requirement of the surgical treatment pattern, the actual focal depth of the laser beam may differ from the desired focal depth for the treatment and an actual focal depth. Hence, there is a need for a closed-loop system that controls movement of a focusing assembly to a desired position and feedback validation that the desired movement of the focusing assembly has been achieved. In this manner, the depth position of a focal point may be precisely controlled.
The present invention relates generally to a closed-loop focal positioning system. More particularly, the invention relates to a method and system for moving a focusing assembly for focusing a laser beam to a desired position (also referred to as the theoretical position) and then determining via a feedback positioning device, an actual movement value of the focusing assembly.
Briefly stated, the closed-loop focal positioning system utilizes a computer processor for the execution of software to control the movement of a focusing assembly used to focus a laser beam. The software is configured to allow an operator to identify a laser focal point or depth. In turn, the focusing assembly is instructed to move to a desired position. A feedback positioning device reads the actual position or movement of the focusing assembly. A comparison of the desired position and the actual position is used to determine if the focusing assembly has been correctly moved, thereby ensuring that the laser beam will be correctly focused when it is activated.
Various laser sources may be used with the inventive system and method, including infrared, visible, and UV lasers. Further, laser sources to be used with the inventive system may be continuous wave, Q-switched pulse, and mode-locked ultrashort pulse lasers. Although the following is not an exhaustive list, lasers of the foregoing type may be used with the present invention. In the preferred embodiment, the laser source is an infrared ultrashort pulse laser with a pulse duration of less than 10 picoseconds.
The focusing assembly may be a single lens, objective lens, compound lens, a lens assembly, curved mirror or series of curved and/or flat mirrors, a combination of the foregoing, or a moveable housing containing the foregoing, that is used to focus a laser beam where the movement of the focusing assembly can be measured and the movement correlated to a focal depth. In other words, the focusing assembly can be any laser focusing device in whole or in part that is moveable, and the movement of such device can be measured by a feedback positioning device to determine if an actual focal point is achieved.
In an embodiment of the inventive system, an operator of a laser system for ophthalmic surgery identifies a desired focal depth position value for photodisruption or ablation of a structure of the eye, such as the cornea or crystalline lens. A software program executing on a host computer receives the value for the laser focal depth position. The software program commands a displacement of a focusing assembly of a laser system by writing a voltage to a Digital/Analog card. A Z Galvo will in turn move the focusing assembly to the desired focal depth position based upon the commanded voltage by directing a current to the motor-driven focusing assembly.
A linear encoder positioned within the laser system senses the linear movement of the focusing assembly. An intelligent controller interoperating with the host computer and software program utilizes a sensor to read an encoder strip attached to the focusing assembly. As the lens is moved into position, encoder feedback is provided by an intelligent controller and an actual focusing assembly position is obtained.
Although in one embodiment the feedback positioning device is a linear encoder, other feedback positioning devices may be used including a rotary encoder, an interferometric encoder, an optical encoder, a resolver, a Heidenheim scale, angular encoders, digital length gauge systems, phase device, magnetic strip reader, or transducer.